Manufacture of light armor plate



Y treated so Patented Feb. 22, 1944 E. I. du Pont de Nemours mington, Del.,

& Company, Wila corporation of Delaware Application February 5, 1942, Serial No. 429,608 claims. (c1. 14s- 1.9)

This invention relates to the manufacture of armor plate and more particularly to the manu facture of light armor plate comprising composite structures of low-carbon and high-carbon steels.

The thickness of armor plate varies ordinarily from one-eighth inch up to the extremely thick plates used for arming battle ships. My herein described invention relates to the manufacture,

of light face hardened armor plate, and I use l0 that term in this specication and in the ap pended claims to mean armor plate not more than about two inches thick.

In light face hardened armor plate it is essential that the outer surface of the armor have a high degree of hardness while the inside or base should be tough and strong. This may be obtained by case hardening steel plate so that a high concentration of carbon is produced in the outer surface of the plate, which produces a hard` surface on heat treatment. In order to produce the required hardness and resistance to projectiles, the case must be of high carbon content and of extraordinary depth. Utilizing conventional case hardening procedures, this requires unusually long time of carburizing treatment, e. g. on the order ofv to 100 hours, or eveil? longer, depending on the thickness of the p a e.

This type of armor plate also may be made by making a so-called ply metal by joining a plate of high-carbon steel to a base plate of low-carbon steel, for example by welding or by the so-called Pluramelt process. This produces a composite or ply steel plate which may be heat as to give extreme hardness to the xterior portion having the higher carbon con- In heat treating ply steel sheets of the type described above, considerable diiculty is often experienced because the'ply metal tends to behave like a bi-metallic structure. This results in warping and distortion as a result of the heating and cooling operations.- The heat treating operations also often .may result in more or less decarburization at the surface of the plate.

An object of the present invention is to provide an improved method for making a composite or ply steel light armor plate. A further objectis to provide a method for preventing distortion or warping from the heat treatment of such ply steel armor plate. Another object is to produce a superior armor plate having improved resistance to projectiles and at the same time, a high degree oi' surface hardness. My invention also includes the improved ply armor plate herein described. Other objects will be apparent from the following description. 1

The appended drawing is a diagrammatic illustration of my invention. Figures 1, 2, 3,v and 4 are graphs showing the per cent of carbon in steel armor plate at different depths, i. e. at different distances irom the outside surface. The depths expressed in decimal fractions of an inch are plotted as abscissae, while the carbon concentrations are plotted as ordinates. Figures 5 and 6 are cross sections of composite steel armor plate of the type herein referred to.

I have discovered that the tendency for ply steel armor plates to warp and distort during heat treatment may be overcome and' the strength and resistance to penetration by projectiles may be greatlyv improved vby subjecting the outside surface of the armor plate to a carburizing treatment while heating the plate to a temperature of 1600 to 1900 F., preferably at around 1750 F. The carburizing treatment selected is one vwhich, is adapted to carburize the steel to produce a carbon case containing substantially the same or a greater percentage of carbon as that already existing in the exterior ply of the composite plate. The treatment is carried out for a suicient length of time to carry a substantial degree of carbon from the exterior ply into the base or low-carbon ply.

The invention may be more readily understood by reference to the drawing. Figure 1 is a graph showing thepercentage of carbon in different depths of a ply-metal or composite plate made by welding a high-carbon steel outer ply to a lowarbon steel base ply. In this example,I

the high-carbon layer4 of the composite plate is about 0.034 inch thick, while the thickness of the entire composite plate is 0.25 inch. Figure 5 is a diagrammatic cross section of such a plate. consisting of the high-carbon outer ply l and the low-carbon base ply 2, these being joined together by weld 3. The graph of Figure 1 shows that the carbon content of the outer ply l is 0.9% substantially throughout except on the surface, while the carbon content of the base ply 2 is substantially 0.24% throughout. It will be noted that the outer surface of tle composite plate is relatively low in carbon content, this being caused by decarburization resulting from hot working in making the composite plate.

Figures 2 and 6 show the application of my invention applied to the plate illustrated by Figures 1 and 5. Figure 2 shows the carbon concentrations in the plate after it has been subjected to a suitable carburizing treatment, as hereinafter described.

This carburizing treatment has resulted in the disappearance of the decarburzed zone shown in Figure 1 at the surface of the plate and has also caused a relatively broad diiusion of carbon from the outside ply into the base ply. Referring to Figure 6, this zone of carbon diusion is diagrammatically represented by layer Referring back to Figure 2, the graph shows the extent of the z'one of carbon diffusion lying between the points D1 and Dz, D1 being the point where the carbon content begins to become smaller than that of the outer ply and Da, the point where the carbon content begins to equal thatof the base ply of the plate. It is also noted that the diffusion of carbon into the base ply has taken place without apparent loss of carbon from the outer ply, the thickness of the outer ply being substantially unchanged. This is because the outer ply has absorbed carbon during the carburizing treatment.

Figures 3 and 4 are graphs similar to Figures l and 2 and represent a two-ply armor plate 2 inches thick. Figure 4, showing a relatively wide carbon diifusion zone (between points Di and D2) illustrates the armor plate of Figure 3 after subjection to the carburizing treatment for a period of approximately 50 hours.

I have discovered that in order to prevent distortion and Working of the plate during heat treatment and, at the same time, to secure the desired ballistic properties, it is essential that there be a definite, limited relationship between (l) the relative carbon contents of the outer ply and the base ply of the composite plate, (2) the width of the above described carbon diffusion zone and (3) the total thickness of the composite armor plate. This relationship may be expressed mathematically, thus:

where D1 and D2 are the depths in inches at the beginning and end, respectively, of the diffusion zone, expressed in inches; C1 and Cz are the carbon contents of the base and outer plies, respectively, of the sheet, expressed in per cent by weight; P is the total plate thickness, expressed in inches; and K is a mathematical constant or numerical factor, which, in accordance with my invention, lies within the range of 0.18 to 0.52, and preferably is about 0.2'0.

For example, for a 1A inch armor plate, the ratio between the diusion zone (D2-D1) and the diierence between the respective carbon contents of the outer and base plies (C2-C1) should be equal to:

Similarly, for a 2 inch armor plate, this ratio will be:

Example I Sheets of low-carbon and high-carbon steel are welded together to form a two-ply composite plate 1/4 inch thick. The composite platev then is held for about 3 hours in a fused salt cyanide case hardening bath containing around 50% by weight of barium chloride, 5 to 10% by weight each of Carbon content of base ply (C1) per cent-- 0.24 Carbon content of outer ply (C2) do.. 0.88 Depth of -outer ply (depth of 0.88% carbon steel, or D1) inch-- 0.034 Depth to base ply (to 0.24% carbon steel,

or D2) inch 0.084

With the diffusion zone (D2-D1) do 0.050

A two-ply plate, 2 inches thick is formed and treated in a base hardening bath according to the method of Example 1, except that the treatment in the case hardening bath at 1750u F. is continued for 48 hours. Carbon gradient analyses give the curves shown in Figures 3 and 4 of the appended drawing, according to the following data:

Ratio Carbon content of base ply (C1) per cent 0.20 Carbon content of outer ply (C2) do 0.38 Depth of outer ply (depth of 0.88% carbon steel, or D1) inch 0.26 Depth to base ply (to 0.28% carbon steel,

or Dz) inch 0.27

Width of diifusion zone (D2-D1) ..do

Ratio D2-D1 0.27 0.435

C2-Cl 0.62 1

Similar results may be obtained by using the various known fused salt -carburizing baths to treat the ply armor plate. For example, there are a number of suitable case hardening baths characterized by containing one or more of the alkaline earth metal chlorides, and up to about 20% by weight (e. g., 5 to 20%) of alkali metal cyanide or other cyanogen compounds, usually also containing some alkali metal chloride, carbonate, or both. An example of this type of carburizing bath which is suitable for practicing my invention is one having the following approximate composition:

Sodium carbonate Remainder While I prefer to use a carburizing bath having the above composition, the invention is not restricted thereto, as various other known types of carburizing salt baths, containing cyanide or other carburizin'g material, may be utilized with equivalent results. Also, other known methods may be utilized, for example pack carburizing or gas carburizing methods. In any case, it is essential that the carburizing medium used shall be capable of producing the desired high-carbon content in the exterior surface of the armor plate, preferably substantially the same carbon concentration as that in the main portion of the exterior layer of the composite plate, or higher. I have found that in most cases these desired vresults are best obtained, and in a minimum salt carburizing bath.

time of treatment by using some type of fused the thickness of the outer ply including the diffusion zone, equal to about 0.2 to 0.25 of the In its broader aspects, my invention includes total plate thickness, a carbon content of about subjecting the ply steel plate to any cementation 0.75 to 1% by weight in th'e outer ply and a cartreatment atv the above stated temperatures, it 5 bon content of about 0.1 to 0.4% by weight in being understood that cementation means the the base ply. In most cases a carbon content introduction of a hardening element into the of 0.22 to 0.28% by weight is preferred for the surface of the steel. For example, the invention base play. The invention is not restricted to may be practiced by a nitriding instead of a cartwo-ply structures, although two plies ordinarily burizing treatment, or by using a. cementation are satisfactory; and itis seldom necessary to methodwhich causes both nitriding and carutilize more than two plies to obtain excellent burizing. Thus, the exterior portion of the armor ballistic characteristics. However, if desired, plate may be nitrided instead of, or in addition light armor plate having morel than two plies to, carburized. In that event, it is essential that can be made according to my invention with good suicient nitrogen be introduced so as to main` 15 results. My invention is not restricted to the use tain the desired hardnesson the surface and in of any particular grade or type of steel; the the outside ply of the treated plate. In other nickel molybdenum alloy steels most commonly words, if the nitriding treatment results in aputilized for light armor plate are well adapted preciable decarburization in the exterior ply, it for my purpose. preferably should introduce sufficient nitrogen In common with other methods for making to produce hardness equivalent to that produced l light armor plate, my herein described carbuby the carbon which is lost. When both carburized ply plate should be heat treated after the rizing and nitriding are utilized, I prefer to first carburization or other cementation treatment, to carburize and then nitride. fully develop the desired physical properties. 'Ihe time required in the carburizing bath or 25 My invention is not restricted to any particular equivalent treatment to cause carbon diffusion methods of heat treatment; the conventional between the plies of the armor plate as above methods may be applied to my ply plate with described Will Very depending upOn factors Such satisfactory results. Generally such heat treatas on the nature of the steel, plate thickness, ment consists in reheating, quenching and drawkind of carburizing or other cementation treating. The details of such heat treatments of ment, treating temperature. methOd 0f heating. course will depend on factors such as the comand the like, as will be apparent to those skilled position of the lsteel utilized and the physical in heat treating metals. The time required may properties desired and are well known to those be determined eXDeIilnentlly in any given CaSeskilled in making steel armor plate.

'essential'that the carburizing or other cementa- Generally, I prefer to hold the armor plate in the My invention results in more rapid manufaccarburizing treatment et e temperature 0f ture of armor plate having excellent physical around 1600 to 1900 F. fOr a period 0f time Varyproperties, which has a high degree of resistance ing With the plate thickness approximately aS to penetration of projectiles and which has com- ShOWn by the following table: paratively little tendency to warp or distort when 40 subjected to heat treatment cycles consisting of Plate thickness 'rggtai heating, quenching and drawing. Heretofore,

similar results could be approached only by case carburizing single sheets of the desired armor plate thickness, whereby it was necessary to carry out the carburizing treatment for exceedingly long periods of time, for example 10 to 100 hours or longer, in order to obtain a case having thi?` required thickness. By means of my process, on the other hand, the composite or ply metal structure can be made relatively quickly and a carburization treatment of 2 to 50 hours, depending on plate thickness, is usually suiiicient. In general, the carburization time in my process is about l@ of the time required to obtain similar results by carburizing a single sheet. Furtherton medium come into Contact only with the ex more, the relatively wide carbon diusion zone tenor Surface of the armor plate; that is, base between the outer and base plies results 1n inonion of the com Osite late must rotected creased strength in the armor plate, and superior from carburizationf) To rodu thisbemt sew balllstic properties. Another important advaneral known methods may be .utilized For ex tage is that the resulting nished plate has the ample, two sheets of armor plate of equal size desired high degree of surface hardness. and shape may be fastened together by placing I claim: them bwk to back and Welding a Seal around 1. The process for manufacture of light armor the edges, After carburizing the sheets may be plate which comprises forming a composite steel separated by cutting them around the edge por- Diete hevme an outer ply of high-carbon steel non or by cutting the Weld- Anm-,her method joined to a base ply of low-carbon steel and sube which is generally satisfactory consists in coppe/'r jecting Said Composite plate t0 a carburizing plating the underside of the composite armor treatment at a temperature of about 1600 to plate, 1900 F. for a time sufficient In the production of ply armor plate in accordance with my invention, the relative thickness and carbon contents of the plies may be A inches, to the difference betweenV the respec varied, depending on the kinds of steel utilized carbon contents of said plies, expressed in and the service fonwhich the plate is intended. cent by weight, is equal to the product of the Generally. I nreferlin a two-ply plate. to have thickness of said plate, in inches. and a numerical The times in the above table are the times during which the metal is at heat and do not include the time required to bring the plate up to the desired treating temperature.

In practicing this invention it is, of course, 55

factor lying in the range: 0.18 to 0.52, while pro tecting the surface of said base ply from contact with the carburizing agent.

2. The process for manufacture of light armor plate which comprises forming a composite steel plate having an outer ply of high-carbon steel ,containing about 0.75 to 1% by weight of carbon joined to a base' ply of low-carbon steel containing about 0.1 to 0.4% by Weight of carbon and subjecting said composite plate to a carburizing treatment at a temperature of about 1600 to 1900 F. for a time sumcient to form at the juncture of said plies a carbon diiiusion zone of such width that the ratio of said width, expressed in inches to the difference between the respective carbon contents of said plies, expressed in per cent by weight is equal to the product of the thickness of said plate, in inches and a numerical factor lying in the range: 0.18 to 0.52, g@

while protecting the surface of said base ply from contact with the carburizing agent.`

3. The process according to claim 1 wherein the plate is treated in a fused salt carburizing bath containing at least one alkaline earth metal salt and up to 20% by weight of alkali metal cyanide.

4. The process according to claim l wherein the plate is treated in a fused salt carburizing bath containing at least 40% by Weight of barium chloride and 8 to 12% by weight of sodium cyanide.

5. In the manufacture of light armor plate, the step comprising subjecting a composite steel plate having joined plies of high and low-carbon steels to a carburization treatment at a temperature of about 1600 to 1900" F.. so as te cause substantial carbon diffusion at the juncture of said plies.

DONALD A. HOLT. 

